The present invention relates to measuring stress and thin film parameters and, in particular, to a method and apparatus that combines initial stress and thickness measurements and optimizes the calculations for both.
There are a number of conventional metrology tools for measuring the characteristics of thin films on semiconductor wafers. Typical parameters of interest are the thickness (t), index of refraction (n), and extinction coefficient (k). The conventional tools employ a variety of approaches for measuring these parameters, including spectrophotometry and ellipsometry. The most advanced and complex tools actually combine a number of these approaches to measure these parameters.
During the fabrication of wafers, thin films are deposited on the wafer. These thin films are typically formed from dielectric or metallic materials. The deposition process often produces residual stresses in the wafer. These stresses can affect the device manufacture and performance. Accordingly, there is an interest in measuring the stress after such a deposition as a means of monitoring or controlling the manufacturing process.
One way to measure stress is to measure the deflection (bow or warp) of the stressed wafer. Typically, one measures the wafer both before and after the stress-inducing manufacturing step. Knowing the wafer thickness, size, stiffness and thickness of the thin film, one can calculate the residual stress from the change in shape. There is already at least one tool on the market which performs stress measurements in this way. Descriptions of this tool can be found in U.S. Pat. Nos. 5,134,303 and 5,248,889, incorporated herein by reference.
Briefly, devices of the type described in the above-cited patents operate by directing a probe beam onto the surface of the wafer. A position sensitive photodetector is then used to measure the location of the reflected probe beam. During calibration, the central portion of the position sensitive photodetector is arranged to coincide with the location where the reflected probe beam would fall (based on Snell""s Law) if the wafer surface was flat. Any bow or tilt in the wafer surface will change the direction of the beam, causing a displacement of the reflected probe beam on the photodetector. The amount and direction of the displacement of the reflected probe beam on the detector provides a measure of the direction and extent of the bow or warp of the wafer.
As noted above, in order to determine the amount of stress based on the measurement of wafer warp, other parameters of the wafer, including wafer size, stiffness and the thickness of the thin film must be known. In the prior systems, a value for thickness used in determining the stress value is selected based on the presumed or desired thickness of the film. However, if the thickness presumption is in error, the calculation of stress based on the measurement of the curvature of the wafer will be inaccurate.
Accordingly, it would be preferable to obtain an accurate measurement of the film thickness before calculating the stress levels in the film. Unfortunately, just as the stress calculation is dependent on the film thickness, conversely, the measurement of the thickness of the film is dependent on the level of stress in the film. For example, stresses in the film can directly affect the index of refraction of the material. While the coefficient of stress induced index change is typically small, the large stresses induced in films during the deposition process (approaching a billion Pascals) can lead to index changes as large as even a few tenths of a percent. If such changes occur and are not accounted for in analysis of the data on thin film measurement, relatively large errors will arise in the computation of thickness and extinction coefficients. As noted above, if the thickness of the film cannot be accurately determined, a determination of the stress cannot be accurately obtained since the level of measured warp can only be accurately converted into stress measurements if the thickness of the thin film is accurately known.
Another parameter which is of interest to semiconductor manufacturers is the birefringent effects of certain films. Many films, such as paralyne, have a natural birefringence. These films (as well as other films) also generate birefringence under stress. It is desired to have a system which distinguishes between the inherent birefringence and the birefringence effect due to stress in the films.
The subject invention which utilizes a composite metrology tool, which measures the basic parameters of thin films (e.g., t, n and k and birefringence) and wafer displacements. These measurements are combined (e.g. in a processor) using optimization techniques to yield accurate overall information of the wafer parameters.